1. Field of the Invention
The present invention relates generally to automobile wiper devices, and more particularly to a variable-driving unit capable of varying the arm pressure and the wiping area of a wiper device, and also to a wiper device incorporating such variable-driving unit.
2. Description of the Prior Art
An automobile wiper device includes a wiper arm driven to oscillate through a segmental or fan-shaped area. The wiper arm is firmly connected at its proximal end to a pivot spindle fixed to one end of a pivot lever pivotally connected at the other end thereof to one end of a link rod, the other end of the link rod being pivoted to a wiper motor. Upon rotation of the wiper motor, the link rod is reciprocated to oscillate the pivot lever and the pivot spindle connected thereto, thereby oscillating the wiper arm. The wiper arm is provided with a wiper blade connected to the distal end thereof for wiping a segmental area on a windscreen to remove raindrops or the like. During oscillating movement, the wiper blade is urged against the windscreen at a predetermined arm pressure by means of an arm spring associated with the wiper arm.
The arm pressure and the wiping area of the conventional wiper device are constant and not adjustable.
It is preferred to maintain the arm pressure at a minimum value while the wiper device is not operated, thereby preventing permanent deformation of the wiper blade. Such minimum arm pressure is not preferable when the vehicle is travelling at high speeds in which instance the wiper blade is subjected to a severe wind pressure tending to lift the wiper blade up from the windscreen against the force of the arm spring, resulting in a substantial reduction of wiping efficiency of the wiper device. This lifting of the wiper blade does not occur when the arm pressure is increased to a certain extent. However, the increased or high arm pressure tends to permanently deform the wiper blade while the latter is being pressed against the windscreen.
As stated above, the wiper blade of the conventional wiper device is oscillated within a predetermined wiping area and this wiping area cannot be changed. This construction is disadvantageous in that the wiper device is likely to be damaged under severe wiping loads acting on the wiper blade and the wiper arm when the wiper blade changes its direction of oscillating movement while the wiper device is operating to wipe the snow on the windscreen. Furthermore, the wiper blade while being oscillated is deplaced laterally outwardly from the predetermined wiping area under a severe window pressure acting on the windscreen when the vehicle is travelling at high speeds. The wiper blade thus displaced tends to slip off the windscreen in a lateral outward direction and damage the vehicle body.
One prior attempt made to overcome the foregoing difficulties caused by the constant arm pressure is disclosed in Japanese Patent Laid-open Publication No. 59-145647 in which an electromagnetic clutch is employed to change the arm pressure. The electromagnetic clutch, however, is difficult to control at an accurate operation timing which is required for spontaneous control of operation of the wiper device. Wiper devices having such electromagnetic clutch are complicated in construction and large in size.